1. Field of the Invention
The invention relates to a device for the incremental scanning of a line scale by means of a number of photocells aligned on the line scale in a pattern offset relative to one another, the output of the photocells being feedable to a counter as electrical impulses.
2. Description of the Prior Art
Devices for carrying out incremental measuring methods for determining the change of position of movable parts are used for positioning tools on machine operators, but more particularly in inclination balances, in which the change in position of a line scale is directly proportional to the weight resting thereon. By the incremental measurement of the position of the line scale a number of impulses are received which are directly proportional to the weight of the goods and which, by means of impulse counters and recording units connected thereto make it possible to numerically record the value of the weight; alternatively, the impulses may be conducted to electronic computers in order to have the measured values processed thereby, so that the price of the goods may be figured out with the aid thereof.
Because of the substantial economic value of such measurements for determining output in commercial transactions it must be demanded that care be exercised so that the results of the measurements are not adulterated by systematic errors which might arise from the breakdown of one of the electronic structural elements used in constructing the measuring equipment.
Various measures have already been made known for protection against errors which arise from the partial or total breakdown of a structural component. For scanning the line scale German Pat. application No. 2,161,634 uses two photocells which are illuminated by a lamp through a line scale and by optical auxiliary means, and which are so adjusted relative to one another that when the line scale moves, attached triggers of small switching point hysteresis deliver impulses in such a way that every impulse that is delivered by one of the two triggers of the first photocell is followed temporally by an impulse from one of the two triggers of the second photocell, and if this condition is not met, a defect signal is given.
German Pat. application No. 2,020,393 tells of another method for controlling defects. In this patent two directionally identified photocells which are adjusted in a special way relative to one another produce counting impulses and the circuit angle and/or the amplitude is checked which, when the scanning signals delivered by the photocells deviate from the required values of the phase position and/or the amplitude, triggers a defect signal.
It is a drawback in both instances that the photoelectric cells must undergo precise adjustment and that, moreover, in the course of carrying out the controlling, changes in the brightness of the lamp or insignificant soiling resulting from the comparison of "required" with "actual" already lead to defect signals. Furthermore, in the second instance the control circuit is a circuit which is separate from the actual discriminator and its breakdown is not monitored.
There is another method wherein at a defined point in the moving scale a third photocell emits an impulse which designates the count as being incorrect if at that very moment it does not coincide with the ones associated with the point in question. The drawback of this arrangement lies in the fact that there is only one point of control in the entire range of movement of the scale, all the other points remaining uncontrolled.